Vegetable gardeners often talk about their plants "bolting," which simply means that the plant sends up a flower stalk and goes to seed. When plants flower, it's generally considered a good thing; however, in vegetables grown for their leaves, such as lettuce, spinach, cabbage and other cole crops, bolting causes the flavor to turn bitter and the leaves to get smaller and tougher, making them inedible. Bolting is common in cool-season greens, like arugula, lettuce and spinach. Other common garden plants that bolt include beets and broccoli and herbs such as cilantro, basil and dill.
Why Lettuce Bolts
Bolting tends to happen when the temperature heats up.
Nevertheless, heat may be a factor in bolting if high temperatures occur when the plants are nearing maturity. Dry conditions may also contribute to bolting. Plants that feel threatened by harsh temperatures will often go to seed. Even exposure to cold while the plants are seedlings can play a role. If lettuce seedlings are exposed to 40 to 50 degrees Fahrenheit temperatures for several days in a row, they will start forming flower buds, although the flower stalk won't shoot up until the weather warms.
The name "iceberg lettuce" dates back to early 20th century California—refrigerators were not available so to prevent spoilage in transit, farmers would cover the lettuce in crushed ice.
Tips to Prevent Bolting
Unfortunately, there isn't much you can do to stop bolting on lettuce and other plants. Plants that bolt tend to thrive in cooler weather, so keeping them growing and edible in the heat of summer will take work.
- Some cultivars are quicker to bolt than others. When you're out shopping for seeds, you may see seeds described as "slow to bolt," but because a variety of conditions can cause bolting, this is no guarantee. That said, if you do have trouble growing lettuce in the summer, it can't hurt to give some of the slow-to-bolt cultivars a try. Many will have names that hint at their bolt resistance, like "Slobolt" and "Summer Bibb." Among the older seed varieties, crisphead lettuces in the romaine or cos group tend to be the slowest to bolt, while looseleaf lettuce tends to be the quickest.
- Many gardeners have had success growing lettuce throughout the summer by planting it in a shaded spot in the garden. You can tuck it behind or under taller plants or grow it in pots that can be moved to a shadier site. Regular watering helps to keep the soil cool and the leaves succulent.
- A good trick for starting seeds in the summer is to use cold water to thoroughly soak the area to be planted about two to three days before sowing. After watering, cover the damp soil with a wide board. Repeat this process daily if the weather is particularly hot and dry. Within a couple of days, the soil under the board will be cooler than the surrounding soil. It's a good idea to test with a soil thermometer to be sure you've achieved the correct temperature for your seeds to sprout. Sow your seeds, and then water and cover the soil again with the board. Check daily for signs of germination. At the first sight of green sprouts, remove the board.
- Shade cloth also can be used to protect young seedlings and tender leafy greens. Most garden centers carry the types of covers specifically designed to let in rain and sun but, at the same time, protect the vegetables under cover from strong UV rays that can cause early bolting. Row covers also protect greens and cole crops from pests such as cabbage loopers and rabbits. You will need to provide supports along with the cover in order to keep the cloth elevated above the plants giving them room to grow.
- If you see your cool season vegetables start to form flower buds, pinch out the buds. Once the plant has begun the process of flowering and reseeding it will eventually bolt, but you may be able to extend your harvest a little longer by pinching out the flower buds early on.